In many wireless networks, a wireless medium may be shared by many different nodes or devices within the network. When multiple nodes transmit on a wireless medium at the same time, collisions may occur that corrupt the corresponding communication. One method for avoiding collisions involves the use of carrier sensing. That is, before transmitting on the wireless medium, a node first senses the medium to determine whether another node is currently transmitting. If another node is transmitting, the first node must wait for a period of time and then try again. If no other node is transmitting, the first node may then proceed with its transmission. While effective, use of such a technique can lead to other problems such as, for example, the hidden node problem. The hidden node problem may occur when there are at least three nodes in the wireless network: node A, node B, and node C. Node B is within range of both node A and node C, but node A and node C are out of range of each other. When node A wishes to transmit to node B, it first senses the wireless network medium and, if no traffic is detected, it initiates its transmission. However, because node C is out of range of node A, it is unable to detect the transmissions of node A. Thus, node C may begin to transmit while node A is transmitting to node B, resulting in a collision that interferes with the communication.
To overcome the hidden node problem, the IEEE 802.11 standard provides a handshaking protocol that allows both node A and node B to reserve the wireless medium for a predetermined amount of time before node A is permitted to transmit user data. When node A wishes to transmit to node B, it first transmits a request-to-send (RTS) frame to node B that indicates the desire to transmit data. The RTS frame also includes a network allocation vector (NAV) that indicates a time period during which the wireless medium is to be reserved. Nodes that receive the RTS frame then record the NAV and refrain from transmitting for the corresponding period. When node B receives the RTS frame, it responds by transmitting a clear-to-send (CTS) frame back to node A that indicates that it is okay to begin transmission. The CTS frame also includes a NAV that reserves the same time period. Because node C is within range of node B, node C receives the CTS frame, reads the NAV, and refrains from transmitting for the indicated period, thus preventing a collision. After node A receives the CTS frame from node B, it may initiate the transmission of user data to node B. After the data has been fully received, node B may transmit an acknowledgement (ACK) frame to node A to indicate that the data has been successfully received.
In the past, the NAVs transmitted by the initiating node and the responding node reserved the wireless medium until the end of the corresponding frame exchange. For example, in the frame exchange described above that includes an RTS frame, a CTS frame, a data frame, and an ACK frame, the NAVs transmitted within both the RTS frame and the CTS frame would protect until the end of the ACK frame. This type of medium reservation technique will be referred to herein as the “long NAV” technique. A problem that arises with long NAV is that, if the initial handshake is unsuccessful, then the medium is still reserved for the entire frame exchange period, even though no data communication will take place, thereby wasting available resources. To deal with this problem, a technique has been proposed that uses NAVs that only reserve the wireless medium until the end of the next transmission of the other communicating node involved in the frame exchange. This type of medium reservation technique will be referred to herein as the “short NAV” technique. The present invention relates to techniques and structures for implementing short NAV type medium reservation in networks that utilize adaptive modulation.